2023 VJC H2 Chem Prelim P3 (Ans)

1 © VJC 2023 9729/03/PRELIM/23 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/03 Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 19 September 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your name and CT group on this cover page. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 / 24 2 / 16 3 / 20 Section B 4 OR 5 / 20 Total / 80 This document consists of 34 printed pages.

2 © VJC 2023 9729/03/PRELIM/23 Section A Answer all the questions in this section. 1 (a) Describe and explain the trend in thermal stability of the hydrogen halides. [2] Thermal stability decreases down the group . Increase in atomic radius from F to I leading to poorer extent of overlap between the orbital of hydrogen and the orbital of the halogen. Hence H-X bond is weaker, less energy needed to break the bond. (b) A sodium salt of the interhalogen ion, IBrx− is prepared. When heated, the salt evolves bromine vapour, leaving sodium iodide as the residue. The bromine vapour evolved requires 32.0 cm 3 of 0.100 mol dm −3 of aqueous sodium thiosulfate for complete reaction. The balanced ionic equation for the reaction between bromine and thiosulfate is as shown below. S2O32−(aq) + 4Br2(g) + 5H2O(l) → 2SO42−(aq) + 8Br−(aq) + 10H+(aq) (i) Construct a balanced equation for the thermal decomposition of NaIBrx. [1] 2NaIBrx → xBr2 + 2NaI (ii) Calculate the amount of bromine gas evolved. [1] Amount of thiosulfate used = 32.0 ÷ 1000 × 0.100 = 3.20 × 10−3 mol Amount of bromine evolved = 4 × 3.20 × 10−3 = 0.0128 mol The sodium iodide residue was completely reacted with concentrated sulfuric acid to give HI gas, which immediately further reacted with conce